The National Institute of Mental Health (NIMH) is studying concentrations of the drug haloperidol in the blood and post-mortem tissue samples of patients with schizophrenia. A sensitive, reliable, and specific assay has been developed to measure concentrations of both haloperidol and its reduced metabolite. One study using this assay has shown that it may avoid some types of interference which complicate radioreceptor methods of determining haloperidol. A basic science study was completed that showed that the reduced metabolite of haloperidol appears not to be pharmacologically inactive. Research using post-mortem brain tissue specimens has indicated that both haloperidol and reduced haloperidol may remain present in detectable amounts as long as 72 days after withdrawal from haloperidol. Clinical studies in patients with chronic schizophrenia have indicated that above a certain threshold concentration of haloperidol (approximately 5ng/ml) in serum, a plateau exists and clinical response is not enhanced by attaining higher concentrations. This implies that in the treatment of chronic schizophrenia there is usually no additional clinical benefit to the use of very high doses of haloperidol.